Hybrid-Electric LightningStrike

Pure electric propulsion isn’t the only route to aeronautical revolution — and maybe not the fastest. Aurora Flight Sciences is working to prove that with its avant-garde LightningStrike, a hybrid-electric technology demonstrator that the Manassas, Virginia, company is building under an $89M Phase II Defense Advanced Research Projects Agency (DARPA) contract. “People are nibbling around the edges of what electric can do,” said John Langford, Aurora’s founder and chief executive officer. “This airplane drives directly to the heart. It transforms the whole configuration of the airplane.”

Aurora’s XV-24A X-Plane

Designated the XV-24A and scheduled for first flight in summer or fall of 2018, LightningStrike is an exotic vertical takeoff and landing (VTOL) aircraft with a tilting 60 ft (18 m) wing on its aft fuselage and a much shorter tilting canard near its nose. Embedded in the wing and canard are 24 ducted fans. Eighteen fans about 30 inches (76 cm) in diameter are in the wing. Six more, these measuring just less than 2 ft (60 cm) in diameter, are in the canard. Each is turned by its own electric motor. But rather than getting their juice from batteries, those motors are powered by three Honeywell generators driven by a Rolls-Royce AE1107C turbine engine — the same engine used on Bell-Boeing V-22 Osprey tiltrotor transports flown by the US Marine Corps, US Air Force and, soon, the US Navy and the Japan Ground Self- Defense Force. LightningStrike’s three 1 MW generators will produce the total electric equivalent of 4,023 hp. “The wing motors are about 125 kW each and the canard motors are each 90 kW, so it’s quite a bit of power,” said Carl Schaefer, Aurora program manager. Aurora’s distributed electric propulsion design was the winning entry in phase two of DARPA’s Vertical Takeoff and Landing Experimental Airplane program, known as VTOL X-Plane. DARPA’s aim is to produce major leaps in four historically difficult performance metrics for VTOL aircraft: speed, useful load fraction, cruise efficiency and hover efficiency. Aurora’s was the only entry incorporating electric propulsion. DARPA asked for a technology demonstrator weighing between 10,000 and 12,000 lb (4.5 to 5.4 t). and able to fly at “sustained speeds” of 300 to 400 kt (555 to 740 km/hr) carrying a “useful load” — fuel, payload and crew — equal to 40% of its empty weight. DARPA made that easier by stipulating that the VTOL X-Plane could be unmanned but requiring that at least 12.5% of total weight be payload. By comparison, a UH-60M Black Hawk helicopter has a primary mission gross weight of 19,398 lb (8,800 kg) and carries a useful load of 6,887 lb (3,125 kg), or 35.5%, according to manufacturer Sikorsky. The VTOL X-Plane also was to fly with about half the aerodynamic drag of standard helicopters and hover at a “figure of merit” — a measure of rotor efficiency — of at least 75%, rather than the more typical 60%. Aurora engineers expect LightningStrike’s distributed electric propulsion configuration, which allows each fan’s pitch to be individually controlled, to provide an extraordinarily agile aircraft. “The endurance of the airplane is about three hours,” said Schaefer. “It will take off, transition to forward flight fairly quickly, and cruise at well over 300 kt (555 km/h). Then the goal is to come to a hover at some notional area of interest, stay there for 15 minutes, and then return home.” Schaefer said his team in Manassas has “already released parts to the factory floor to build the airframe, the wing and canard. The goal is to have an airplane on its gear and ready to roll out in October of this year.” The fuselage will be more than 38 ft (11.6 m) in length and, sitting on its landing gear, LightningStrike will stand about 14 ft (4.3 m) high, Schaefer said. As of early 2017, Honeywell was preparing to test the megawatt generator. “That will all be married into what we call the Iron Bird Test Rig at the beginning of the fall” at Rolls-Royce, Schaefer said. The Iron Bird will test the power generation system from the turboshaft through the drive shaft, the gearbox, the three generators, and on to six wing fans and two canard fans — a third of the system, Schaefer said, “from the AE1107 engine to the fan blades.”

Scaling Up for Success

Aurora has already built and flown several times in hover and slow forward flight the 20% subscale demonstrator (see “A Small LightningStrike,” Vertiflite, July-Aug 2016). This largely 3D-printed Subscale Vehicle Demonstrator (SVD) has LightningStrike’s configuration but uses a pure electric propulsion system. In January, the company took the subscale demonstrator to a Navy test field, where Schaefer’s team was to “finish up the transition to high speed flight” over a period of several weeks. A six-month flight test program is planned for the full scale LightningStrike once it takes to the air in 2018. Schaefer said Aurora hopes DARPA, NASA or other government agencies will be intrigued enough to fund continued testing, or perhaps pursue prototype drones, or even manned aircraft based on LightningStrike. “There are some conceptual drawings of a manned version of these,” he said. “A utility version similar to a Black Hawk. There’s also a manned version of an armed escort for a V-22. Again, these are conceptual designs, but we have given it some thought.”

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